1. Field of the Invention
This invention relates to an X-ray tube apparatus which can output X-rays of a dose suitable for radioscopy for a long time.
2. Description of the Related Art
In fields of medical diagnosing apparatuses and non-destructive testing apparatuses, X-rays are widely used in obtaining an image of an object to be tested, that is, an object of a photograph. If, for example, a still picture of X-ray image of an object is to be obtained, intensifying screens and films are mainly used. If, for example, moving image information is to be obtained, an X-ray image tube (X-ray detector) is used.
These days, in a method of imaging an object by using an X-ray image tube, two filaments having different focuses are used, and X-rays of a radioscopic dose with a small focus are applied to the object to obtain moving image information thereof. In the meantime, a method is widely used in which X-rays of a large dose with a large focus for still pictures are applied to the object to obtain a still picture thereof, under specific conditions or in the screen a picture of which is to be obtained.
For example, Jpn. Pat. Appln. KOKAI Pub. No. 2002-83560 has already proposed a rotating anode X-ray tube having a filament 21a with a large focus and a filament 21b with a small focus.
Further, Jpn. Pat. Appln. KOKAI Pub. No. 6-290721 has already proposed a rotating anode X-ray tube, in which two filaments 3 are provided on respective focusing grooves 7 with an anchor 4 interposed therebetween.
These days, when moving images of the object are obtained by applying X-rays of a radioscopic dose with a small focus by using the above X-ray image tube, it is desired to obtain an image having a maximum resolution even in moving images.
However, when a current supplied to the filament of a small focus is increased to provide a radioscopic dose, there is the problem that the operation temperature of the filament rises and thereby the life of the filament is sharply shortened.
This increases the running cost of the medical diagnosing apparatuses and non-destructive testing apparatuses into which the X-ray tube is integrated, since it is required to change the X-ray tube before the filament of the large focus for still pictures reaches an end of its life. In particular, in medical diagnosing apparatuses, there are cases where it is impossible to suspend the test and to take a waiting time, and the problem cannot be solved by simply changing the filament (or X-ray tube apparatus).